FIELD OF THE INVENTION
The invention relates to a metal-ceramic gradient material, a product made from a metal-ceramic gradient material, in particular a thermal shield or a gas turbine blade, and a process for producing a metal-ceramic gradient material.
In order to provide heat-resistance for a component which is exposed to extremely high temperatures, for example a thermal shield or a gas turbine blade, it is known, for example from U.S. Pat. No. 4,321,311, to produce the component from a metal base body and to coat the metal base body with a ceramic thermal barrier layer of ZrO.sub.2. The ceramic thermal carrier layer is bonded-on through a metallic adhesion promoter layer formed of an alloy of the type MCrAlY. Since the ceramic thermal barrier layer is generally a good conductor of oxygen ions, the adhesion promoter layer becomes oxidized during the course of operational use of the component. As a result, the thermal barrier layer can become detached from the metal base body. That results in the duration of use of a component of that kind being limited. That is the case in particular in the event of frequent temperature changes, which occur when a gas turbine is started up and run down.
An article entitled "Keramische Gradientenwerkstoffe fur Komponenten in Verbrennungsmotoren" [Ceramic Gradient Materials for Components in Internal Combustion Engines] by W. Henning et al. in Metall, Vol. 46, Issue 5, May 1992, pp. 436-439, discusses a fiber ceramic body with a density gradient for the purpose of improving the resistance of piston heads to temperature changes. That fiber ceramic body is composed of four layers of different layer thicknesses and different ceramic fractions. The difference in the ceramic fraction resides in the fact that the ratio of fibers (short Al.sub.2 O.sub.3 fibers) to ceramic particles of Al.sub.2 TiO.sub.5 differs distinctly in the four layers. As a result, the porosity of the four layers also differs considerably with respect to one another. The high porosity of the layers, lying between 40% and 79%, is used to introduce molten metal into the cavities in the fiber ceramic body through the use of squeeze casting, so as to produce a defect-free composite. As a result, it is possible to produce a piston head which has a gradient of metal and ceramic that changes considerably and suddenly. Due to the low thermal conductivity of the ceramic fractions, a thermal barrier is formed, so that the piston is insulated. In addition, the fiber ceramic reinforces the piston and therefore improves the resistance of the piston to thermal shocks.
An article entitled "Projected Research on High Efficiency Energy Conversion Materials", by M. Niino and M. Koizumi in FGM 94, Proc. of the 3.sup.rd Int. Symposium on Functional Gradient Materials, ed. B. Ilschner and N. Cherradi, pp. 601-605, 1994, has described composite materials, in connection with the development of materials for a space shuttle, which are referred to as Functional Gradient Material (FGM). The essential feature of FGM is a continuous composition gradient and/or microstructure gradient, which is intended to lead to a continuous gradient of the relevant functions, e.g. the strength, thermal conductivity, ductility, inter alia.
The intention is to increase the load-bearing capacity and efficiency of the material by avoiding abrupt changes in properties. FGMs are therefore intended to combine the positive properties of laminated composites and lump composites in a single material.